Monolithic piezoelectric accelerometers and other piezoelectric accelerometers are known. Examples of such prior art piezoelectric accelerometers are seen in the following U.S. patents:
______________________________________ U.S. Pat. No. Date To ______________________________________ 5,461,918 1995 Mozurkewich 5,193,392 1993 Besson et al. 5,138,414 1992 Shinohara 4,924,131 1990 Nakayama et al. 4,670,092 1987 Motamedi ______________________________________
Accelerometers have a wide range of application, including sound and vibration monitoring, and the detection and measurement of velocity changes. Hence a great many accelerometer sensor types have been devised, developed and made commercially available.
Each type of accelerometer has characteristics which make it more or less suitable for a particular application. For example, geophones are highly sensitive at very low frequencies; hence they are excellent for monitoring earthquakes. Conversely, miniature solid-state accelerometers are very cheap, quick, and easy to use. While they have very poor sensitivity, they fit the needs of sensors for triggering air bags.
The broadest commercial class of accelerometers is the longitudinal type. This type fills the requirements falling between the above extremes. Longitudinal type accelerometers have good sensitivity over a wide frequency band, and are rugged enough for general purpose industrial use. Longitudinal type accelerometers are the key component for most machinery diagnostic equipment, inclinometers, and vibration monitoring. There are many major manufacturers of this type of accelerometer, such as the PCB (registered trademark) piezoelectric accelerometers by Piezotronics Co. and Model 793 Piezoelectric Accelerometer by Wilcoxon Research, of Rockville Md. The scope of the present invention is intended to be limited to this class of piezoelectric accelerometers, the important high-volume commercial type accelerometers.
Further, conventional piezoelectric accelerometers have a mass supported by a piezoelectric element with electrodes At frequencies much below the spring-mass resonance of the system, the seismic mass exerts a stress on the piezoelectric support which is proportional to the acceleration of the system. Under such a stress, the piezoelectric element delivers a charge or voltage to the electrodes which is proportional to the stress. Hence the output charge or voltage is proportional to the acceleration of the system.
For a high quality measurement accelerometer, such a conceptually simple configuration presents some fabrication and operation difficulties. In a typical embodiment, the accelerometer is sealed in a pressure-tight metal container to reduce the effect of environmental pressure variations. This metal container also shields against stray electrical and magnetic fields. To reduce the influence of temperature fluctuations, it may be surrounded by a thermal mass. Alternative arrangements of the mass and piezoelectric elements effects have been devised to reduce interfering effects such as nonlinearities due to high acceleration, and the sensitivity to base strain. Because of the many components involved, the fabrication process can be labor intensive.
For example, one application is in 1-3 composite transducers. Such transducers include a dense array of piezoelectric rods embedded in a soft matrix. The Naval Research Laboratory has a long and continuing interest in the development of accelerometer arrays, particularly for underwater acoustic velocity sensing. Such arrays are formed from commercially available accelerometers.
Commercially available measurement accelerometers are typically constructed in multiple labor-intensive fabrication operations which result in high cost.